Project 3 - Abstract Three interrelated but independent hypotheses are being tested in this project. A.) Aptamers can serve as molecular probes to identify and study functionally important exosites on coagulation factor cofactors FVa and FVIIIa; B.) Aptamers targeting specific regions identified within FV and FVIII can serve as procoagulants by allosterically activating FV or preventing the binding of FVIII to vWF; and C.) Exosite binding Factor V and VIII aptamers will limit thrombosis in vivo and antidote oligonucleotides can be made to rapidly reverse the anticoagulant activity of such aptamers to restore normal hemostasis in animals. Our specific aims are: Aim 1: To generate anticoagulant aptamers against coagulation factors Va and VIIIa and use them as novel probes for exosite interactions with the cofactors within the respective enzyme complexes. Aim 2: To generate novel procoagulant aptamers targeting the procofactor FV and FVIII Aim 3: To evaluate the ability of aptamers targeting exosites on Factor V/Va and VIII/VIIIa to act as potent yet safe antithrombotic agents.